Ultra high energy cosmic rays from axion stars

We propose a model in which ultra high energy cosmic rays are produced by collisions between neutron stars and axion stars. The acceleration of such a cosmic ray is made by the electric field, ∼1015 (B/1012 G) eV cm−1, which is induced in an axion star by the strong magnetic field B>1012 G of a neutron star. As we have shown previously, similar collisions generate gamma ray bursts when the magnetic field is much smaller, e.g. ∼1010 G. If we assume that the axion mass is ∼10−9 eV, we can explain huge energies of the gamma ray bursts ∼1054 erg as well as the ultra high energies of the cosmic rays ∼1020 eV. In addition, it turns out that these axion stars are plausible candidates for MACHOs. We point out a possibility of observing monochromatic radiations emitted from the axion stars.